Targeting early metastable intermediates of the SARS-CoV-2 spike for vaccine and therapeutics development The ongoing global pandemic of the novel SARS-CoV-2 coronavirus (CoV) presents an urgent need for development of effective preventative and treatment therapies. The viral-host cell fusion (S) protein spike is a prime target for such therapies owing to its critical role in the virus lifecycle. The S protein is divided into two regions: the N-terminal S1 domain that caps the C-terminal S2 fusion domain. Binding to host receptor via the Receptor Binding Domain (RBD) in S1 is followed by proteolytic cleavage of the spike by host proteases. This leads dramatic conformational transitions resulting in S1 shedding and exposure of the fusion machinery in S2, culminating in host-cell entry. Class I fusion proteins such as the CoV S protein that undergo large conformational changes during the fusion process must, by necessity, be highly flexible and dynamic. Indeed, cryo-EM structures of the SARS-CoV-2 spike reveal considerable flexibility and dynamics in the S1 domain, especially around the RBD that exhibits two discrete conformational states ? a ?down? state that is shielded from receptor binding, and an ?up? state that is receptor-accessible. The overall goals of this study are to use our robust, high-throughput computational and experimental pipeline to define the detailed trajectory of the ?down? to ?up? transition of the SARS-CoV-2 S protein, identify early metastable intermediates in the fusion pathway, and exploit their structures and dynamics for identifying drug and vaccine candidates that target SARS-CoV-2. A wealth of structural information on CoV spike proteins, including recently determined cryo-EM structures of the SARS-CoV-2 spike, provides a rich source of detailed data from which to begin precise examination of macromolecular transitions underlying triggering of this fusion machine. The scientific premise of this study is that understanding the structural dynamics and early transition kinetics of mobile regions of the SARS-CoV-2 spike will allow optimal control of vaccine and drug responses, and facilitate the development of novel antiviral drugs and protective vaccines. At the culmination of this study, we expect to have determined structures of multiple ?down?, ?up?, and intermediate states of the SARS-CoV-2 S protein. Together, these studies will provide important atomically detailed structural and mechanistic information for exploitation in vaccine and therapeutics design.